Hydraulic power units are widely used in a variety of manufacturing applications. These hydraulic power units include a hydraulic reservoir sump tank which serves as a receptacle for holding hydraulic fluids such as well known petroleum based oils. These hydraulic units are closed looped systems in which fluid is pumped from the tank, generally by an electrically operated hydraulic pump and supplied to one or more valves in a valve deck. The outputs of these valves are connected to various hydraulicly energized devices such as cylinders with pistons therein which move in accordance with the amount of hydraulic fluid applied to one side or the other of a piston head therein. Fluid in the non-pressurized side of the hydraulic cylinder is forced out of the cylinder. In conventional hydraulic power units, the outlet lines from the hydraulic cylinders or other devices are connected to return lines for carrying the fluid back to the tank. These return lines have in the past merely passed through comparatively large holes in the top of the sump tank. Consequently, the entire interior of the tank was open to atmospheric pressure.
It is known that conventionally used hydraulic fluids may contain a certain amount of gaseous impurities. For example, it is common to find an appreciable amount of air in petroleum oils. These gaseous impurities can affect the accuracy of the hydraulic positioning devices operating under control of the pressurized fluid. If the hydraulic fluid is not completely homogeneous, but contains dissolved air therein, the system response will be affected since the impurities in the fluid cause it to be somewhat compressible or "spongy". In sophisticated, finely tuned hydraulic systems such as in robotic applications, it is imperative that the hydraulic fluid be non-compressible and free from impurities so that the accuracy of the positioning devices will not be impaired.
The prior art recognizes that subjecting hydraulic fluid to a vacuum will serve to draw off the impurities therein. See, for example, U.S. Pat. No. 3,357,161 to Starr et al., and U.S. Pat. No. 3,193,988 to Kudlaty. However, the apparatus employed in these and other prior art systems do not lend themselves to incorporation into hydraulic power units of the type described above which use sump tanks as reservoirs for the hydraulic fluid in the system.